A typical electrophotographic image forming apparatus has an electrostatic latent image bearing member for bearing electrostatic latent images thereon. In the image forming operation, the outer peripheral surface of the electrostatic latent image bearing member is electrically charged with a uniform potential by a charger. The uniformly charged surface is exposed to image light to form an electrostatic latent image. The electrostatic latent image is visualized by a developer or toner supplied from a developing device into the toner image. The visualized toner image is transferred onto a recording medium such as paper directly or through an intermediate transfer member in the form of endless belt. Then, the recording medium is transported into a fixing device where the toner image is press-heated and thereby fixed onto the recording medium.
There has been proposed a color image forming apparatus in which a plurality of color toner images are firstly transferred and superimposed sequentially from the electrostatic latent image bearing members onto the belt and then the superimposed toner images are secondly transferred onto the recording medium. The second transfer roller is disposed adjacent the outer peripheral surface of the belt. When a certain transfer voltage is applied between the belt and the second transfer roller, an electric field is generated between the belt and the second transfer roller to electrically bias the toner from the belt toward the transfer roller. This results in that the charged toner images are transferred onto the recording medium passing through the contact region between the belt and the second transfer roller.
Disadvantageously, some foreign matters such as insufficiently charged toner particles fly away into the atmosphere at the first transfer regions. The flying foreign matters may be caught on the outer peripheral surface of the belt and then transferred onto the second transfer roller. The foreign matters on the second transfer roller are then transferred onto the recording medium passing through the second transfer region.
To overcome this problem, JP 2002-82537 A and JP 2005-49449 A, for example, disclose a technique in which a cleaning or scraping blade is disposed in contact with the outer peripheral surface of the belt. According to this technique, the foreign matters are scraped off from the belt by the blade, which prevents the foreign matters from being transferred onto the second transfer roller or the recording medium.
However, this technique has a drawback that, in order to completely remove the foreign matters from the belt by the scraping blade, the blade should be pressed so strongly onto the belt, which in turn deteriorates durabilities of the belt and the blade.
Considering this problem, it can be thought to use a roller made of a metal core and a polyurethane foam layer covering the peripheral surface of the metal core for the cleaning member for removing the foreign matters from the outer peripheral surface of the belt.
The property of the polyurethane foam may determine a cleaning efficiency and/or a cleaning ability. For example, it may be important to adjust the open ratio of cell walls and the number of cells per unit length of the polyurethane foam for increasing the cleaning property of the polyurethane foam layer. The above-mentioned JP 2002-82537 A discloses to use the cleaning blade together with the cleaning roller, however, it is silent about the property of the polyurethane foam including open ratio of cell walls or the number of cells per length.
The present invention is to provide a cleaning roller, for use with the transfer member and an image forming apparatus using the cleaning roller, which is capable of maintaining the initial cleaning ability for a long period of time.